Recently, a nonaqueous electrolyte battery such as a lithium ion secondary battery has been actively researched and developed as a high energy-density battery. The nonaqueous electrolyte battery is expected to be used as a power source for hybrid vehicles, electric cars, an uninterruptible power supply for base stations for portable telephone, or the like. For this, the nonaqueous electrolyte battery is desired to have a high energy density as well as to be excellent in other performances such as rapid charge-and-discharge performances and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge-and-discharge not only remarkably shortens a charging time but also makes it possible to improve performances related to motivity and to efficiently recover regenerative energy from motivity, in a hybrid vehicle or the like.
In order to enable rapid charge-and-discharge, electrons and lithium ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, dendrite precipitation of metal lithium occurs on the electrode, raising the fear as to heat generation and fires caused by internal short circuits.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in the negative electrode has been developed. Particularly, in a battery using titanium-containing oxide as the negative electrode active material, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than those using a carbonaceous material.
However, an electrode containing titanium-containing oxide has a higher operating potential based on metal lithium than an electrode containing the carbonaceous material. That is, the electrode containing the titanium-containing oxide is nobler. Furthermore, titanium composite oxide has a lower capacity per weight. Therefore, a battery using titanium composite oxide as the negative electrode active material has a problem that the energy density is lower. Particularly, when a material into which lithium ions are insertion and from which lithium ions are extracted at a high potential based on metal lithium is used as a negative electrode material, a battery using the material has a lower voltage than that of a conventional battery using a carbonaceous material. Therefore, when the battery is used for systems requiring a high voltage such as an electric vehicle and a large-scale electric power storage system, the battery has a problem that the battery series number is increased.
The operating potential of the electrode containing titanium-containing oxide is about 1.5 V based on metal lithium and is higher (nobler) than that of the negative electrode using carbonaceous material. The potential of titanium oxide is due to the redox reaction between Ti3+ and Ti4′ when lithium is electrochemically inserted and extracted, and is therefore limited electrochemically. It is therefore conventionally difficult to drop the potential of the electrode to improve the energy density.